It is clear that Corynebacteriophage beta carries the structural gene for diphtheria toxin, tox. Tox is only expressed by lysogens of Corynebacterium diphtheriae which are starved for iron. As yet, little is known about the molecular mechanisms which regulate the tox gene; however, there is good evidence which suggests that the bacterial host plays a major role in the regulation of the phage tox gene. I have been interested in the regulation of tox expression and the role that iron plays in this process. I have approached this problem in two ways: the first being the in vitro synthesis of diphtheria tox gene products in Escherichia coli S-30 extracts, and secondly by the isolation of both beta-phage and corynebacterial mutants by which the Tox character is altered. Each of these methods has provided additional support for the hypothesis that C diphtheriae, lysogenic or non-lysogenic, contain a protein with tox "repressor" activity. The long range objectives of this investigation are to understand the molecular mechanisms that are involved in the regulation of the beta-phage tox gene both in vitro and in vivo.